This invention relates in general to a vehicle braking system and in particular to the use of two state actuators for braking wheeled vehicles.
Vehicles are commonly slowed and stopped with hydraulic or pneumatic braking systems. These systems vary in complexity but typically include a pressure vessel, such as master cylinder, for pressurizing the hydraulic fluid or air, fluid conduits arranged in two or more separate brake circuits, and wheel brakes connected to each circuit.
Typical wheel brakes include a rotor or drum having a friction section with friction surfaces disposed thereon. The rotor or drum is attached to one or more of the braked vehicle wheels for rotation therewith. The wheel brakes also include one or more brake pads or brake shoes having friction material disposed thereon. The brake pads or shoes are located adjacent the rotor or drum friction surfaces. One or more pressure actuators are secured to a non-rotating component of the vehicle, such as the vehicle frame, at each braked wheel. The pressure actuators typically include a wheel brake cylinder having a moveable piston operatively connected to one or more of the brake pads or brake shoes.
When the driver brakes the vehicle, the pressure vessel pressurizes the fluid in one or more of the brake circuits. The fluid pressure is transmitted through the conduits to the pressure actuators. The pressure actuators apply translational forces against the pads or shoes which in turn press against the friction surfaces of the rotating rotor or drum. As the brake pads or shoes press against the moving friction surfaces, braking forces including frictional forces are generated which oppose the rotation of the wheels and slow the vehicle.
Advances in braking technology have led to the introduction of Anti-lock braking or ABS systems. An ABS system monitors the rotation of the wheels and maintains the wheel speed within a selected slip range to achieve maximum braking forces. Pressure regulating devices, such as valves, selectively relieve and reapply pressurized fluid in the circuits communicating with the corresponding wheel brake cylinders to control the wheel speed.
Further advances have led to the introduction of Stability Control systems which improve the stability of the vehicle by using braking forces to counteract excessive cornering forces. These forces and other vehicle parameters are detected by sensors which signal an electronic control unit to automatically operate selective pressure regulators, such as valves. The pressure regulators control the fluid pressure applied to the pressure actuators thereby generating the desired braking forces without the driver applying the brakes.
These known braking systems can be characterized as pressure proportionate braking systems since the magnitude of the fluid pressure received by the pressure actuators determines the magnitude of the braking forces produced at the respective wheels. The pressure actuators can be actuated to a plurality of different states in which the pressure actuator in each state provides a different translational force to the brake pads or shoes. The translational force applied to the brake pads or shoes determines the braking forces generated at the rotor or drum and the associated vehicle wheel. These pressure controlled braking systems require complicated pressure regulating controls to achieve the precise fluid pressures at the pressure actuators needed for maximum braking performance under all conditions.
It would be desirable to provide a braking system which achieves the desired braking performance in all operating conditions without the need to achieve such precise pressure control of conventional pressure controlled braking systems.